Wireless communication systems are known to enable one wireless communication device to transmit data to at least one other wireless communication device via a wireless transmission medium. Such wireless communication systems may range from National or International cellular telephone systems to point-to-point in-home networking. For instance, a wireless communication system may be constructed, and hence operates, in accordance with one or more standards including, but not limited to, IEEE 802.11a, IEEE 802.11b, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wireless application protocol (WAP), local multi-point distribution services (LMDS), multi-channel multi-point distribution systems (MMDS), and/or variations thereof.
As is also known, such wireless communication systems use radio frequencies for the wireless transmission medium. Thus, each wireless communication device that transmits data requires an RF transmitter and each wireless communication device that receives data requires an RF receiver. In general, an RF transmitter includes a modulator, local oscillator, one or mixers, power amplifier and an antenna. The inter-operation of these components is well known to modulate a data signal into an RF signal. Correspondingly, an RF receiver includes an antenna, which may be shared with the RF transmitter, a low noise amplifier, a local oscillator, one or more mixers, a summing module, filtering, and a demodulator to recapture the data signal from the RF signal.
Consequently, each wireless communication device includes a plurality of mixers within the RF transmitter and RF receiver to properly function within any type of wireless communication system. Not surprisingly, the quality of performance of a wireless communication device is dependent on the quality of performance (e.g., linearity) of the mixers included therein. A high quality mixer for certain applications is illustrated in FIG. 1 and is known as the Gilbert mixer. The Gilbert mixer, as shown, may be implemented using standard CMOS technology, however, for low supply voltage applications (e.g., less than 3.3 volts), it is difficult to obtain sufficient gain due to the output resistors R0 and R1.
To overcome this limitation, the Gilbert mixer can be modified as shown in FIG. 2. While this configuration improves the headroom capabilities of the mixer, it still has some limitations. For instance, such a mixer lacks built-in gain control, which would allow the gain of the mixer to be adjusted for various applications. In addition, the mixer, when used to directly translate RF signals into base-band signals, includes a significant amount of flicker noise, which is produced by switching transistors MG1-MG4. Further, the voltage excursion on switching transistors MG1-MG4 may be quite large, which causes device reliability issues of the switching transistors. Still further, the maximum swing of the mixer output, while maintaining acceptable distortion performance, could be quite limited for low supply voltage applications (e.g., less than 2 volts).
Therefore, a need exists for a mixer that reliably operates at low voltages (e.g., less than 2 volts), provides gain adjustments, reduces adverse affects of flicker-noise and/or limits voltage excursions of its switching transistors, which improves device reliability.